1. Field of the invention
The present invention relates to a frequency modulator. More specifically, the present invention relates to a frequency modulator which is utilized in a video or sound signal recording circuit, and etc. of a VTR, for example, and is constituted by a monolithic integrated circuit (IC).
2. Description of the prior art
One example of a well-known frequency modulator is shown in FIG. 2. As shown in FIG. 2, a frequency modulator is generally constituted such that a modulation signal is added to a control voltage for a voltage controlled oscillator (VCO). In addition, a frequency-modulated signal (FM signal) can be represented by a center frequency Fo and a frequency deviation amount .DELTA.f in a case where the modulation signal like a sound signal includes no direct current components.
Then, assuming that an oscillation frequency of the VCO is F, the following equation (1) is obtainable. EQU F=Kt.multidot.V (1)
where, V is a control voltage and Kt is a conversion coefficient.
If the above described equation (1) is divided into the center frequency Fo and the frequency deviation amount .DELTA.f, since the center frequency Fo and the frequency deviation amount .DELTA.f are corresponding to a direct current control voltage Vo and a modulation signal .DELTA.v, respectively, the equation (1) can be modified as follows: EQU F=Fo+f=Kt(Vo+.DELTA.v) (2)
Normally, due to variations of characteristics of an oscillation capacitor and a coil within the VCO and temperature drift thereof, the equation (2) can be modified as follows: EQU F=Fo+.DELTA.f=Kt(Vo'+.DELTA.v')=Kt(.alpha.Vo+.beta..DELTA.v)(3)
In the equation (3), Vo'=.alpha.Vo and .DELTA.v'=.beta..DELTA.v, and therefore, in the frequency modulator as shown in FIG. 2, it is necessary to respectively adjust the direct current control voltage Vo and the modulation signal .DELTA.v so as to correct the conversion coefficient Kt by correction values .alpha. and .beta. so that the center frequency Fo and the frequency deviation amount .DELTA.f become target values, respectively.
In the frequency modulator of the above described prior art, it is necessary adjust or control the center frequency Fo and the frequency deviation amount .DELTA.f. By an application of an APC (Automatic Phase Control) by a PLL (Phase Locked Loop), the center frequency Fo can be maintained in a crystal accuracy without any adjustment; however, an adjustment of the frequency deviation amount .DELTA.f is still indispensable to such a frequency modulator, and therefore, there were necessity of a component for adjusting the frequency deviation amount (normally such a component is variable resistor) and an adjustment step thereof.